A composition includes a calcium carbonate-comprising material and from 0.1 to 8 wt.-%, based on the total weight of the calcium carbonate-comprising material, of at least one grafted polymer comprising at least one succinic anhydride group obtained by grafting maleic anhydride onto a homo- or copolymer including butadiene units and optionally styrene units and/or salty reaction products thereof a polyester mixture including the composition, a polyester product prepared from the polyester mixture, a process for preparing the polyester product as well as the use of the at least one grafted polymer to decrease the melt flow rate of such a polyester product and an article formed from the polyester product.